SX-42 Bandswitch

[Joe Connor]

Can someone give me some troubleshooting help regarding the bandswitch on an SX-42? I'm working on an SX-42 now. It has FM to die for but the lower four bands are very weak. I've replaced the caps and out-of-spec resistors in the RF section. The voltages there appear pretty good. Nevertheless, the weak AM reception persists. I've examined the switch the best I can but don't see any burned spots etc.

Joe Connor

[W1EUJ]

Have you seen the restoration that Phil Nelson did? see antiqueradio.org.

[WQ9E]

Joe,

If it is only the 4 lower bands then the problem could be in the 455 Khz. IF strip (open winding, shorted cap, etc.) or there could be a problem with the selectivity switch.  If you have a signal generator, I would start by injecting a 455 KHz. signal into the IF strip near the detector and move towards the front end.  Use just enough signal at first to be detectable and the detected audio should increase as you move towards the front of the IF strip.  If it suddenly drops then you have found the problem stage.

I wouldn't spend a lot of time in the RF stages until you are sure the IF strip/detector is working properly.

Rodger WQ9E

[w3jn]

The silver mica capacitors inside the IF transformers are prone to failure.  I had to replace all of them in a SX-42 I did; the symptom is extremely low IF gain and an inability to get a sharp peak on the 455 IF transformers.  You don't hafta remove the transformers, just remove the cover.  If the FM is working FB don't worry about those in the 10.7 MHZ IF section.

Also I sure hope you replaced all the capacitors associated wtih the tone control switch.  One shorts to ground and it takes out 1/2 of the audio output transformer.  That transformer bears checking - if half the primary is already burned out (as most are) one of the output tubes is getting no plate voltage but full screen voltage.  Not good.

[Joe Connor]

This set is definitely a challenge. I did some signal injection and the IF is strong and gets stronger as I move back from the 2nd IF. All paper caps are gone. The voltages on the 6V6s are fine. The set receives on all bands. THe problem is low gain and sensitivity on the lower four. Also, when I switch the AVC off, I notice no difference. I'm wondering if my next step is to check out both the Selectivity and Reception Mode switches.

[w3jn]

NO.

Read my post above.  Check the IF gain at each stage.  There's a Sams Photofact avaialble on BAMA I believe that indicates stage gain.

If you can't find that, try peaking the 455 IF cans.  If you don't get a nice sharp peak on each one, the silver mica inside has gone bad.

Put the selectivity switch in narrow/non-crystal.  Throw a 455 KHz signal of about 100 microvolts or so on the rotor of the mixer section of the variable cap.  If you don't get a decent signal, the problem's in the IF.

There may be an AVC issue, or there's not enough IF amplification to generate much AVC action.  But let's start with the IF stages.

Be advised there are at least 3 different schematics out there for this and I don't think I've owned a SX-42 that fully complied with any of them (I've had more than 5 over the years, can't remember exactly how many).  The Halllicrafters schematics are hard to follow; the Sams is a bit better.

[Joe Connor]

OK, I went into the IF strip and there are problems. The IF will not peak sharply. I guess it's time to order some silva-mica caps. I assume that if I undo the two nuts under the chassis, the cover of that IF can will lift off. Anything else I need to know before tacking those cans? I've done cans in BC-779s before but never in a Hallicrafters.

Joe Connor

[w3jn]

Yep, that's it.  The silver micas are soldered in, don't bother unsoldering, just clip the leads and do a pigtail repair.  You'll wreck the small wires coming from the coil if you try unsoldering.  Ask me how I know this 

[N3DRB The Derb]

Yep, that's it.  The silver micas are soldered in, don't bother unsoldering, just clip the leads and do a pigtail repair.  You'll wreck the small wires coming from the coil if you try unsoldering.  Ask me how I know this

yeah...I've done that on various rx's over the years.

nothing makes me hotter than discovering that the great deal I got on a >insert receiver here<was because it had a open tuned coil impossible to repair, which was why the guy let it go so cheap...... memories...... 

[Don, W2DL]

Hi Joe,

If you want to take a ride out to  Blairstown I'm sure I can make up proper
cables. I've got a LOT of stuff to make them with, including the proper
connectors to the equipment and the test probes. You'll need to bring the
equip. along to make sure the meter has the correct input resistor for DC
readings.

Give me a call at 908 362 8502  -  I'll be here all day Sunday and Monday
after 1 pm or so, other days after 8:00 PM is good.

Don, W2DL

[KM1H]

Thats a 3 year old thread, I think Joe had it fixed long ago

Carl